A digital camera is a type of mobile electronic unit which is fundamentally different than a conventional film-type camera. The digital camera photographs a subject through an optical lens system and stores photographed image information as digital data in a memory card. The digital data stored in the memory card can be processed using a computer and can be easily supplied via a network. Accordingly, the demand for digital cameras is expected to substantially increase.
In order for a digital camera to operate, a power source must be supplied from an AC adapter or a battery. If an AC adapter is connected to a digital camera in which a battery has already been installed, the digital camera uses the power supplied from the AC adapter because a ground is connected to the negative terminal of the AC adapter. If the AC adapter is removed from the digital camera in which a battery has been installed, the digital camera uses the power supplied from the battery because the ground is connected to the negative terminal of the battery. Since the AC adapter cannot be removed at the same time the battery power source is applied, the ground is in a floating state during the time period while the AC adapter is being removed. During this time period, the battery source voltage applied to the digital camera is rapidly reduced. Until the AC adapter is completely removed from the digital camera and the power source is supplied from the battery (several to several tens of ms), the digital signal processing unit is in an unstable state, and the LCD screen displays an abnormal image. Accordingly, a need exists for a stable reset function to simultaneously reset the digital signal processing unit and the controller in a mobile electronic unit digital when the power source is changed from an AC adapter to the battery in the mobile electronic unit.
In FIGS. 1 and 2, a conventional camera power supply unit 14 that generates source voltages is connected to a power source through a fuse 13. For example, first and second source voltages Vd1 and Vd2 are supplied to a digital signal processing unit 15, and a source voltage Vm is supplied to a controller 16, specifically a microcontroller. The camera power supply unit 14 uses the power source supplied from an adapter 11 or a battery 12. A reset unit 17 determines whether the source voltage Vm for the controller 16 supplied from the camera power supply unit 14 is abnormal and resets the controller 16 if the source voltage Vm is abnormal.
However, in FIG. 2, a DC 5V power source is supplied from the adapter 11 at periods b and c where the ground is in a floating state, that is, during a transition period between removing the adapter 11 and applying the power source from the battery 12. During the transition period, the voltage is reduced to 0V at a steep slope during period b and is maintained at 0V during period c. The interval of periods b and c varies according to the speed at which the AC adapter 11 is removed from the digital camera. After period c, a power source Vbatt, supplied from the battery 12, is applied to the digital camera.
The source voltage Vm applied to the controller 16 is not reduced as rapidly as the DC 5V power source supplied from the adapter 11. The source voltage Vm is maintained at an original voltage until part of the way into period c, but then is reduced, and finally restored to the original voltage at a predetermined period of time after period c. The first and second source voltages Vd1 and Vd2 of the digital signal processing unit 15 start to be reduced during period b, but are restored to the original voltage at a predetermined period of time after period c.
As described above, the respective source voltages Vd1, Vd2, and Vm of the digital signal processing unit 15 and the controller 16 are not simultaneously reduced due to a communication error between the digital signal processing unit 15 and the controller 16 during the transition period between removing the adapter 11 and applying the power source of the battery 12. This causes several problems to occur. During period c, the controller 16 operates normally because the fall in power source voltage is small. However, the digital signal processing unit 15 is reset and then starts operating again without an initialization process which causes the LCD screen to display an abnormal image.